The present invention relates to a reflection hologram capable of being viewed by looking through it. More particularly, the present invention relates to a transmissively viewable reflection hologram which allows an illuminating light source for holographic reconstruction to be placed behind it.
Recently, holograms have been put to practical use actively, and both volume and relief holograms have been used for various applications.
The volume and relief holograms may be summarized as follows.
Volume holograms
Reflection type: The
The diffraction efficiency is high, and the angular selectivity and the wavelength selectivity are also high. A bright and clear color image can be reconstructed. However, an illuminating light source needs to be placed on the viewing side of the hologram. The illuminating light source may interfere with the design when the hologram is used for a shop-window advertisement (billboard) or the like.
Transmission type:
High chromatic dispersion occurs, and a color image is viewable only within a narrow angle range.
Relief holograms
Reflection type:
Because a transparent vacuum-evaporated film is provided on the reverse side thereof, the hologram is practically a transmission hologram. Accordingly, high chromatic dispersion occurs, and a color image is viewable only within a narrow angle range.
Transmission type:
High chromatic dispersion occurs because of a transmission hologram, and a color image is viewable only within a narrow angle range.
FIG. 5 is a graph showing comparatively the angular selectivity and wavelength selectivity of a transmission type volume hologram and a reflection type volume hologram [Toshihiro Kubota, xe2x80x9cAn Introduction to Holographyxe2x80x9d, FIG. 2.17 (Asakura Shoten)]. It will be understood from the graph that the reflection type (Lippmann hologram) exhibits extremely high wavelength selectivity and has low chromatic dispersion in comparison to the transmission type, although these are the same in terms of the hologram structure, i.e. volume hologram.
It will be understood from the foregoing that it is desirable to use a reflection type volume hologram to reconstruct a bright and clear color three-dimensional image without introducing rainbow-like dispersion into the reconstructed image.
However, when a reflection type volume hologram is used, it is necessary in order to reconstruct the recorded image by using an illuminating light source to place the illuminating light source in front of the hologram, that is, on the viewing side of the hologram. Therefore, the illuminating light source may interfere with the viewer.
In view of the above-described problems associated with the prior art, an object of the present invention is to provide a transmission hologram which allows an illuminating light source for holographic reconstruction to be placed behind the hologram, i.e. on the side opposite to the viewing side, which permits reconstruction of an image that is bright and clear and has low chromatic dispersion, which is the feature of the reflection type volume hologram.
To attain the above-described object, the present invention provides a transmissively viewable reflection hologram including a reflection type volume hologram member and a semitransparent reflecting mirror or a semitransparent reflecting layer placed at either side of the reflection type volume hologram member. The semitransparent reflecting mirror or the semitransparent reflecting layer is integral with or separate from the reflection type volume hologram member.
In this case, the semitransparent reflecting mirror or the semitransparent reflecting layer may be placed at the illuminating light entrance side or the viewing side of the reflection type volume hologram member.
The distance or angle between the reflection type volume hologram member and the semitransparent reflecting mirror or the semitransparent reflecting layer may be adjustable.
The semitransparent reflecting mirror or the semitransparent reflecting layer may be variable in transmittance.
In the reflection hologram according to the present invention, a semitransparent reflecting mirror or a semitransparent reflecting layer is placed at either side of a reflection type volume hologram member such that the semitransparent reflecting mirror or the semitransparent reflecting layer is integral with or separate from the reflection type volume hologram member. Therefore, an illuminating light source for holographic reconstruction can be placed behind the reflection hologram, i.e. on the side opposite to the viewing side. Moreover, the hologram according to the present invention can be used as a transmission hologram capable of reconstructing an image that is bright and clear and has low chromatic dispersion, which is the feature of the reflection type volume hologram.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.